Fuel Tank Shape

LAST EDITED ON Jul-25-04 AT 08:15 AM (CDT)
 
Is it better to have a true wedge shape fuel tank or a square tank with a triangle shape on one side? The space for my fuel tank on my new airplane is 1.25" high, 2" wide, 3.5" long from front to back. I was planning to build a metal tank that is 1.25" tall, with parallel 1" parallel sides (top and bottom) to the center of the tank then taper down to a point or triangle shape to the edge where the fuel line picks up the fuel. Maybe it would be better to build the tank 1.25" tall triangle shaped 2" wide with no parallel sides a true wedge shape. Is it best to glue the tank in rigid or use some padding?

In response to the first part of your question, it doesn't matter. If you use a square tank with a wedge at the outside end you get the most fuel capacity. The wedge needs only to be at about a 60 degree angle (less out, more up) as you will (or should have) at least twice the outward "pull" as the force of gravity. The fuel then actually sits at an algne in the tank and will empty all the way.

Putting the tank in solidly is better than padding it and allowing it to flex (and the fuel to foam). But it would also be better to make it removeable and slightly less than the height of your compartment so that you can adjust it up or down as needed.

What fuel capacity do you need? A one inch high tank two inches wide will give about one ounce capacity per inch in length. A full one and a half inch high tank will give about one and a half ounces per inch in length. ("About", guys, "about".)

LAST EDITED ON Jul-25-04 AT 08:49 PM (CDT)
 
I looked at airplane pics on the internet and pictures of airplanes in stunt news. I see only one large profile airplane with a metal fuel tank all the other large profiles have gone to new technology using the plastic bottle type fuel tank with a clunk. I can not see the fuel tanks inside the full fuselage airplanes and the old times stunt and small profiles are the only airplanes with metal tanks. I found a video of a profile airplane that has a camera mounted just behind the fuel tank. The video shows the airplane flying the pattern and you can see the clunk and the fuel going up and down inside the plastic tank as the airplane turns. After watching the video I think I will use a clunk tank on my airplane.

>I looked at airplane pics on the internet and pictures of
>airplanes in stunt news. I see only one large profile
>airplane with a metal fuel tank all the other large profiles
>have gone to new technology using the plastic bottle type
>fuel tank with a clunk. I can not see the fuel tanks inside
>the full fuselage airplanes and the old times stunt and
>small profiles are the only airplanes with metal tanks. I
>found a video of a profile airplane that has a camera
>mounted just behind the fuel tank. The video shows the
>airplane flying the pattern and you can see the clunk and
>the fuel going up and down inside the plastic tank as the
>airplane turns. After watching the video I think I will use
>a clunk tank on my airplane.

You can do that, and that is fine. I think you will find that there are more metal tanks than clunk tanks in use, but they each have their supporters. On the video that you saw, the plastic tank was needed in order to see what happens to the fuel. And, yes, the clunk will go up and down. But if you look carefully you should also see that the fuel in the tank is at a slant due to gravity's pull "down" and centrifrugal force's pull "out". The slant will shift as the airplane goes from upright to inverted and back again, but the angle will remain the same. With the metal tank, the wedge shape will be equal to or greater than the angle of the slant of the fuel. And the pickup tube, being in the center of the wedge, will thus always be under the fuel until it runs out. If you plumb these in a uniflow fashion, the pressure on the fuel will also remain relatively constant until the uniflow tube vent uncovers at the very end of the run. This then makes a very smooth run from beginning to end of the pattern. There are various methods for adding uniflow to a clunk tank, but I, personally, have had my problems with these. They have their advantages in being cheap and you being able to see what is going on. The disadvantages are clunks can hang up, tubes can pull off or develop holes, the cork ends can spring leaks. Matal tanks can rust over time, They can get "gunk" inside if you don't rinse them out before long time storage, and they, too, can spring leaks if not soldered properly. Still, I prefer the metal, others prefer the clunk.

I built a metal rectangle tank, 1.125" x2" x3.375". I used a fuel line hole inside the tank with a clunk that I made. The hose is pushed on both brass fuel lines 3/4" of hose slide up on each. Its not going to come off very easy because I made sure this one won't fall off. I had a metal clunk tank once before that seemed to work great until the hose fell off inside the tank.

I have been working on rectangular steel clunk tanks for some time now. I had one in a large profile model at the 97 NATS. It works perfectly. It gives me my best runs and allows me to fill the tank with whatever amount of fuel I want and still draw fuel. This combined with a vent plumbed right behind the prop works better than anything else I have.
It would seem to me that it would allow the maximum amount of fuel in any given compartment.

"SAVE ALL THE PIECES"-Phil Brown